A typical wireless local area network (WLAN) comprises one or more basic service set (BSS). The BSS comprises an AP with at least one WTRU associated to the AP. Typically, in a system with multiple BSSs, (and thus multiple APs), there can be great differences between the quality of the radio connection a WTRU experiences based on which AP the WTRU associates with. Similarly, the radio resources available to a WTRU depend highly on which AP to which the WTRU is associated with. The system capacity can be optimized by managing which AP each WTRU associates and reassociates with during the system operation.
The association of the WTRU and the AP is performed at the instant the WTRU joins the WLAN, (i.e., initial association). AP reselection can take place after the initial association. There are typically two reasons for the AP reselection. First, the WTRU may have moved such that the WTRU can now be better served by another AP. Second, it may be desirable for the WTRU to be served by another AP because the radio resources used by the currently serving AP are strained, (i.e., the channel used by the serving AP is congested), while a neighboring AP has spare radio resources, (i.e., its channel is less congested). It should be understood that the Load Balancing is therefore the management of appropriate points of attachment for WTRUs while roaming inside the WLAN network.
The AP reselection may be driven by either the WTRU or the network. In the WTRU-driven AP reselection, each WTRU is responsible for determining what criteria the WTRU would use for AP reselection. Because different WTRUs may use different policies, and these policies may be geared toward maximizing the quality perceived by only one WTRU rather than the quality perceived by the whole system, WTRU-driven AP reselection is not considered to perform optimally for load balancing. However, because it is the WTRU that first experiences the quality of the radio connection from the candidate APs, WTRU-driven AP reselection allows for minimal signaling and high reaction speed which is useful in cases that the WTRU is moving rapidly or that the conditions of the radio channel vary rapidly.
In the network-driven AP reselection, the AP reselection process is controlled by the AP. This allows for consistent load balancing policies across all WTRUs in the network and theoretically offers the biggest performance gains from load balancing. However, the network-driven AP reselection may require a great amount of signaling overhead and may impair the ability of the WTRU to perform quick AP reselection when needed.